You may already be aware from the Observatory's weather forecast that a tropical cyclone may enter the northern part of the South China Sea next week and bring squally heavy showers to the coast of southern China. Some of you may even notice that certain computer models such as that from the European Centre forecast a direct hit of the tropical cyclone on Hong Kong. Nevertheless, there were discrepancies among different computer models as regards the forecast track. Some predicted a tropical cyclone making landfall over the eastern part of Guangdong while some forecast a tropical cyclone edging close to western Guangdong. Even for the European Centre itself, different members of its ensemble model forecast vastly different tracks (Figure 1).
Figure 1 The ensemble forecast results of European Centre on 28 July 2016, inclusive of forecast tracks from some 50 members. Numbers circled represent the number of days counted from the date of forecast.
Where a tropical cyclone makes landfall, e.g. to the east or to the west of Hong Kong, can result in significantly different local weather. For instance, if it lands to the east, Hong Kong will be affected by northerly winds for a while. Due to terrain effect, the wind strength over Hong Kong will be lower. In case it skirts to the south of Hong Kong and makes landfall to our west, local winds will not weaken significantly. The prevailing southeasterly winds will even push sea water towards the coast and result in storm surge. Coincidently the new moon occurs next week. There is a chance that the astronomical high tide will increase the threat of flooding brought by storm surge. The Observatory will closely monitor the impact of tropical cyclone on local weather. Please stay tuned to our weather updates.
L.S. Lee and Y.C. Chan
Have you come across a Very Hot Weather Warning icon on the TV screen early in the morning with a temperature of just 28 degrees displayed next to it? How come such temperature is considered as "very hot"?
In fact, forecast is a main component of the Very Hot Weather Warning. Even if the temperature at the Observatory is only 27 or 28 degrees in the morning or the weather is relatively cloudy, forecasters may still issue or maintain the Very Hot Weather Warning when they forecast more sunshine leading to a significant rise in temperature. This is to remind the public to get well-prepared before they depart home, such as wearing suitable clothing, bringing sufficient drinks and sunscreen.
Sometimes the weather is very hot during the day with relevant warning in effect but the temperature at night only drops slightly (say 28 degrees or above, which is known as a "hot night"), the forecaster usually will not cancel the warning at night if very hot weather is forecast to recur the next day. This is to avoid confusion caused by the frequent changes in warning status.
The Hong Kong Observatory takes into consideration several factors when issuing the Very Hot Weather Warning. These include the atmospheric conditions, weather situations over different parts of Hong Kong, and the Hong Kong Heat Index which reflects the combined effect of temperature, humidity, wind speed and solar radiation. When the warning is in force, we should take appropriate actions to prevent heat strokes.
T.S. Tsoi & L.S. Lee
While Super Typhoon Nepartak (Figure 1) brought havoc to Taiwan last week, it was fortunate that Hong Kong remained safe from its direct impact. Although no Tropical Cyclone Signal No. 1 was necessary, our forecasters had been keeping a vigilant eye on the evolution of Nepartak.
Figure 1 Satellite imagery at 1 p.m. on 7 July, showing the distinct eye of Nepartak (Source: Japan Meteorological Agency)
As early as 4 July, the Hong Kong Observatory (HKO) began to issue the forecast track of Nepartak and predicted that it would land in the middle part of Taiwan on 8 July. At that time, other meteorological centres and some computer forecasts (including the European Centre (EC) and the National Weather Service (NWS) of USA) forecast that Nepartak would hit the northern part of Taiwan or even just skirt across the offshore waters. Thereafter, the Observatory revised the forecast track in the morning of 6 July, predicting that Nepartak would land over the southern part of Taiwan. This turned out matching quite closely the actual situation (Figure 2).
Figure 2 Forecast tracks of Nepartak by different meteorological centres
Why did the forecasts of various meteorological centres and computer forecasts differ?
The relatively westerly track forecast by the Observatory was mainly based on the consideration that the subtropical ridge of high pressure over the western Pacific would extend westwards from 4 July to 6 July, and thereafter should remain over there for some time. The flow at its periphery would steer Nepartak to keep its west-northwest track. As such, forecasters chose a relatively westerly track among the computer forecasts. As it turned out, even though the main body of the subtropical ridge weakened on 8 July (i.e. the day when Nepartak landed over Taiwan), a weak ridge still persisted over Taiwan and caused Nepartak to maintain its west-northwest movement (Figure 3).
Figure 3 Background field of geopotential heights at 500 hectopascals level (after removing the circulation of Nepartak), depicting the extension of subtropical ridge over Taiwan
It was likely that the eastward bias of the forecast tracks of the other centres was due to the eastward-biased tracks of different computer forecasts (Figure 2). As it turned out, forecast tracks of computers (e.g. European Centre) gradually shifted westward day by day from 4 July to 7 July. Only until 7 July did the computer forecast track become steady and resemble the actual conditions (Figure 4).
Figure 4 Forecast tracks of Nepartak from the European Centre from 4 July to 7 July
Some weather enthusiasts may recall a number of occasions where computer forecasts displayed such systematic bias. Instead of relying too much on computer forecasts, we should take into account all available data and forecast products, and integrate them with our forecasting experience plus analysis of historical cases. Nonetheless, many complicated weather phenomena still remain unsolved by scientists. Forecasters still need to endeavour to deal with each and every challenge of such "unpredictable weather".
L.S. Lee & C.M. Shun